Keep detailed notes, and when you really can't stand it any longer consult with a local Family Law attorney. If she disparages you in front of the children, or withholds visitation, you may have grounds for a Petition. However, until it affects your relationship with the kids, you are better off trying to be thick skinned about it.
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Review your final decree, and if applicable, any subsequent modifications to your visitation and custody arrangement. Are there any provisions relating to no overnight adult guests or live-ins when the children are present? Telling you to cave is a mind game, but in itself won't get far with a court, so the advice of thick skin is very appropriate. Denial of visitation or ignoring other aspects of the custody and visitation terms is another matter. Just don't try to make a small molehill into the Catskills. Judges tend to really not like overstated claims.